Addition type thermoset polyimide resins are finding increased use as matrix resins for advanced composites, adhesives and coating applications. Many different approaches have been explored to obtain resins and composites which are easy to process but better in mechanical strength and temperature resistance. The fastest growing area of addition-type polyimides is that of bismaleimides (BMI) which are synthesized by condensation of maleic anhydride with aromatic diamines and subsequent imidization. The most commonly used building block in BMI chemistry is bis(4-maleimidophenyl)methane (1): ##STR1## When homopolymerized, highly crosslinked and highly aromatic backbones result producing a very thermally stable but brittle matrix. Considerable formulation work has been carried out to improve the toughness and processability while retaining high temperature properties. To achieve these goals, bismaleimides have been blended and coreacted with allyl/vinyl monomers (Stenzenberger et al, 29th Nat. SAMPE, p.1043 (1985)), thermoplastics (Yamamoto et al, 30th Nat. SAMPE, p. 903 (1985)) and reactive rubbers (Shaw et al, Int. J. of Adhesion and Adhesives, Vol. 5, p. 123 (1985)). The best results have been obtained with allyl phenyl type coreactants (Stenzenberger, British Polymer J., Vol. 20, p. 383 (1988)).
One new system consists of bis(4-maleimidophenyl)methane and O,O'-diallylbisphenol A of the formula (2): ##STR2## Upon heating the mixture at 110.degree.-125.degree. C. it forms a homogeneous prepolymer which has a low viscosity (.about.500 cP at 100.degree. C.) and can be processed into prepreg and composites with the existing equipment and technology used for epoxy resins. Upon curing, a crosslinked polymer is formed which exhibits excellent high temperature properties with significant improvement in toughness. However, for the development of other significant promising techniques like resin transfer molding and/or filament winding, it is necessary to reduce the viscosity of the prepolymer and the processing temperature. Preferably, the resin should have a viscosity of 200-300 cPs at 70.degree.-80.degree. C. [Robertson, British Polymer J., Vol. 20, p. 417 (1988)].
Accordingly, it is an object of the present invention to provide reactive diluents which maintain the high temperature resistance and mechanical strength of the crosslinked bismaleimide based polymer.
It is a further object of the present invention to provide curable formulations comprising bismaleimides and substituted diallyl phenols exhibiting a viscosity of about 200-300 cPs at about 70.degree.-80.degree. C.
Various other objects and advantages of this invention will become apparent from the following description thereof.